There exist a number of gene polymorphisms on the genome, which have been considered to be deeply associated with susceptibility to diseases, individual variations in drug metabolism, and the like. The detection of the gene polymorphism is indispensable for so-called tailor-made medicine and becomes one of the most important subjects on the clinical applications of genomic science. Among others, much interest is lately focused on SNP (single nucleotide polymorphism; gene polymorphism caused by substitution of a single base) as a marker of the gene polymorphism, on which huge research funds have been spent on a global basis. On the other hand, data on gene mutations associated with various genetic diseases has been accumulated into databases by virtue of progress on molecular genetics research. Accordingly, it has become reality to make the diagnosis of genetic diseases or the prediction of clinical categories by screening for known gene mutations already found to be pathogenic on the basis on these databases. In particular, a gene mutation that occurs with high frequency within a certain population or interracially is of great diagnostic value.
The gene polymorphism and gene mutation include, for example, a base substitution, deletion, insertion, and variations in the number of repetitive sequences, and among them, a point mutation caused by substitution of a single base makes up the overwhelming majority. A method of simply and quickly detecting a point mutation is indispensable for applying the outcomes of human genome research to clinical purposes.
Until now, a variety of methods have been devised for detecting a point mutation (see Cotton R G H. Mutation Detection. pp. 1-198, Oxford University Press, Oxford, 1997). Typical methods include the allele specific oligonucleotide hybridization (ASO) method, allele specific amplification method, restriction enzyme digestion method, ligase chain reaction, and minisequencing method. These methods require complicated procedures including hybridization or electrophoresis after DNA amplification. On the other hand, the TaqMan method, invader assay, DNA microarray (DNA chip) assay, TOF-MASS method with the use of a mass spectrometer, and the like, which have been recently developed for promoting the human genome analysis and research, are suited to deal with a large number of samples. However, these methods require high-priced, specialized instruments and cannot be easily performed at clinical laboratories. Alternatively, the SSCP method, chemical cleavage method, and DHPLC method are widely used for screening of gene mutations, and are highly effective for broad screening of unknown gene mutations; but are inadequate to reliable detection of a known mutation. In addition, the detection of a point mutation by the use of the sequencing method requires complicated procedures and high expenses, and is of undeniably too much quality for the detection of a known mutation. At present, all of these methods described above involve special examinations performed at gene research laboratories and find a great difficulty in quick performance in clinical settings (or at bedside).
Probes used in the ASO method has been conventionally 15 to 25 mer (see Saiki R K, Erlich H A. Detection of mutations by hybridization with sequence-specific oligonucleotide probes. In: Mutation Detection: A Practical Approach. pp. 113-129, IRL Press, Oxford, 1998). Moreover, it is known that the specificity of a labeled probe for hybridization is enhanced using an oligonucleotide that competes with the probe (see Nozari G. Rahbar S, Wallace R B. Discrimination among the transcripts of the allelic human β-globin genes βA, βS and βC using oligodeoxynucleotide hybridization probes. Gene 43: 23-28, 1986).